This invention relates to methods of sand consolidation in subterranean formations. More particularly, the invention relates to improved methods for selectively consolidating the surface of sand grains in-situ using site specific catalysts.
Sand consolidation is a near wellbore treatment of a producing well. Surrounding a wellbore in many instances are highly porous and fragmentable sand formations. Under production conditions, the sand is often displaced from its aggregated structure and carried along by a fluid flood operations to a producing well. If the sand flow is allowed to proceed unchecked the producing wellbore soon becomes full of sand, thereby clogging oil production. Furthermore, sand arriving at the surface of the well wears out the production hardware.
It has therefore been the subject of extensive research by the petroleum industry to develop techniques to minimize or obviate displacement of sand particles into producing well areas. One such general approach suggested in the art is to consolidate the porous sand structures. Sand consolidation techniques are aimed at cementing loose sand structure adjacent a wellbore. Such consolidation is necessary to prevent breakdown of sand formations and subsequent clogging of producing wells.
The major feature of sand consolidation is to make a sieve next to the producing wellbore such that oil can reach the wellbore while sand flow is retarded. The sieve is composed of sand formations already present surrounding the wellbore area cemented together by consolidating material added to the formations. The consolidation need only be thick enough to prevent sand breakdown adjacent to the wellbore. Ordinarily sand consolidation is effected for six to eight inches around the periphery of the wellbore.
Accordingly, the criteria for good sand consolidation are as follows. First, the consolidation structure should hold back sand particles while preserving high permeability. As consolidating material is added to sand formation, this material will necessarily occupy part of the pore space causing permeability reduction. Therefore, an object of practical sand consolidation is to minimize occlusion of pore spaces so that oil can flow to producing wellbore.
Second, durability is a desired feature of sand consolidation. The thin film of consolidating material holding the sand grains should be physically durable in resisting deterioration by oil, water, and other fluids in the formation. In particular, the consolidated structure should be resistant to high temperature degradation induced by steam displacement operations.
Third, the consolidation action should not set up in the wellbore prematurely thereby occluding the wellbore path for oil production.
Further, the consolidation operation should be simple, efficient, and inexpensive. The economic significance of such an operation is great and the problems described above have been well recognized by the petroleum industry for many years. Many efforts have been made to satisfy these problems in whole or in part.
One particular approach to sand consolidation has been to inject into a well a polymerizable resin which polymerizes within the well formation upon exposure to the elevated well temperature. The polymer material forms a viscous mass which readily adheres to the porous sand structures. As the sand surfaces become coated, they no longer are subject to displacement when exposed to fluid flow. Unfortunately, the polymerization reaction is difficult to control so as to only consolidate the sand grains without plugging the pore spaces and ultimately blocking permeability through the porous strata.
One of the better sand consolidating agents for high temperature wells is a polymerized furfuryl alcohol resin. The material resists heat as well as oil and water. The problem in using this material is in catalyzing the polymerization. If delayed action catalyst is carried in a mixture containing the furfuryl alcohol oligomer, polymerization may occur too early causing wellbore blockage or too extensively so that permeability is lost; or alternatively, polymerization may not occur at all or be inadequate to effect consolidation.
The technique described by the present invention represents an improvement over the methods used previously, in that polymerization and consequently consolidation of the sand positively occurs and occurs only adjacent to the sand grains, thus preserving most of the original permeability. The polymerization reactions disclosed are effected by an acidic material and happen only where the acidic material is present. By injecting a solution of an acidic salt catalyst, the salt selected as one which is adsorbed by sand or rock, the catalyst is present exactly where needed.